Support dissecting REAL (BER) data values

Both exponent and 'integer N' values are limited:
 * max exponent is 3 octets/24-bits
 * max integer N is 8 octets/64-bit

Tested with zero value/length, integers, doubles, positive and negative numbers all using the Basic Encoding Rules (BER)

Change-Id: If92e1b3e209c42909b8cb76e6f50b8e6cd1da0da
Reviewed-on: https://code.wireshark.org/review/5527
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>

1 files changed, 69 insertions, 4 deletions

diff --git a/epan/asn1.c b/epan/asn1.c
index 1ce1bdcb47..b878c719b8 100644
--- a/epan/asn1.c
+++ b/epan/asn1.c
@@ -196,24 +196,89 @@ rose_ctx_t *get_rose_ctx(void *ptr) {
   return rctx;
 }
 
-double asn1_get_real(const guint8 *real_ptr, gint real_len) {
+/** Only tested for BER */
+double asn1_get_real(const guint8 *real_ptr, gint len) {
   guint8 octet;
   const guint8 *p;
   guint8 *buf;
   double val = 0;
 
-  if (real_len < 1) return val;
+  /* 8.5.2    If the real value is the value zero,
+   *          there shall be no contents octets in the encoding.
+   */
+  if (len < 1) return val;
+
   octet = real_ptr[0];
   p = real_ptr + 1;
-  real_len -= 1;
+  len -= 1;
   if (octet & 0x80) {  /* binary encoding */
+    int i;
+    gboolean Eneg;
+    gint8 S; /* Sign */
+    guint8 B; /* Base */
+    guint8 F; /* scaling Factor */
+    gint32 E = 0; /* Exponent (supported max 3 octets/24 bit) */
+    guint64 N = 0; /* N (supported max 8 octets/64 bit) */
+
+    guint8 lenE, lenN;
+
+    if(octet & 0x40) S = -1; else S = 1;
+    switch(octet & 0x30) {
+      case 0x00: B = 2; break;
+      case 0x10: B = 8; break;
+      case 0x20: B = 16; break;
+      case 0x30: /* Reserved */
+      default:
+        /* TODO Add some warning in tree about reserved value for Base */
+        return 0;
+    }
+    F = (octet & 0x0c) >> 2;
+
+    /* 8.5.6.4 Exponent length */
+    lenE = (octet & 0x3) + 1;
+    if(lenE == 4)
+    {
+      /* we can't handle exponents > 24 bits */
+      /* TODO Next octet(s) define length of exponent */
+      DISSECTOR_ASSERT_NOT_REACHED();
+    }
+
+    Eneg = (*p) & 0x80 ? TRUE : FALSE;
+    for (i = 0; i < lenE; i++) {
+      if(Eneg) {
+        /* 2's complement: inverse bits */
+        E = (E<<8) | ((guint8) ~(*p));
+      } else {
+        E = (E<<8) | *p;
+      }
+      p++;
+    }
+    if(Eneg) {
+      /* 2's complement: ... and add 1 (and make negative of course) */
+      E = -(E + 1);
+    }
+
+    lenN = len - lenE;
+    if(lenN > 8)
+    {
+      /* we can't handle integers > 64 bits */
+      DISSECTOR_ASSERT_NOT_REACHED();
+    }
+    for (i=0; i<lenN; i++) {
+      N = (N<<8) | *p;
+      p++;
+    }
+    val = (double) S * N * pow(2, F) * pow(B, E);
+#ifdef DEBUG
+    printf("S = %d, N = %lu, F = %u, B = %u, E = %d -> %f\n", S, N, F, B, E, Eneg, val);
+#endif
   } else if (octet & 0x40) {  /* SpecialRealValue */
     switch (octet & 0x3F) {
       case 0x00: val = HUGE_VAL; break;
       case 0x01: val = -HUGE_VAL; break;
     }
   } else {  /* decimal encoding */
-    buf = ep_strndup(p, real_len);
+    buf = ep_strndup(p, len);
     val = atof(buf);
   }